The work on dose-response of mammalian cells in tissue culture under oxygenated and under anoxic conditions led to calculations of the dose required to cure human tumors. It was immediately apparent that much larger doses would be needed than are used clinically. The conclusion was that anoxic portions of these tumors must become oxygenated during the course of a therapy series. Several aspects of reoxygenation invite investigation. Is a time ever reached when all of the anoxic cells have become oxygenated? Does the size or spacing of the fractions have any influence on the degree to which reoxygenation takes place? A method of examining these questions was suggested by an experiment reported over a year ago. Spontaneous mammary adenocarcinomas in C3H mice were treated with 1,000 R twice a week while the mice breathed air or carbogen. The tumors were measured at the time of each treatment by fitting them to graduated holes in stiff paper. These measurements are plotted on semi-log paper in Figure 1 and sh...